The researchers interviewed 475 prostitutes in five countries - the United States, South Africa, Thailand,
Turkey and Zambia.

Rape is common

They found that 62% reported being raped, 73% said they had been assaulted and 68% said they had been threatened with a weapon.

The severity of the post-traumatic stress disorder suffered by the prostitutes was comparable to that of Vietnam veterans, the researchers said.

The study was led by Melissa Farley, a psychologist at Kaiser Permanente, one of America's biggest health maintenance organisations.

It was released at a meeting in San Francisco of the American Psychological Association.

Trauma is intrinsic

Streetwalking is highly dangerous

The researchers interviewed mostly prostitutes working the street because they were the ones easiest to find.

But while they found "significantly more physical violence in street, as opposed to brothel, prostitution, there was no difference in the incidence of post-traumatic stress disorder."

"This suggests that psychological trauma is intrinsic to the act of prostitution," the researchers said.

Psychologists began the study to help decide whether prostitution amounts to a violation of human rights.

Veronica Monet, a 38-year-old prostitute who works in San Francisco, took issue with the findings.

"My job is stressful and my job has bad days, but it's way better than anybody else's I know," she said.

"The most stressful thing about my job is that it's illegal."

Dangerous and degrading

But Julie Bindel, of the Research Centre on Violence, Abuse and Gender Relations at Leeds Metropolitan University in the UK, said the research provided important evidence that prostitution was a dangerous and degrading activity.

She said: "Prostitution is an extremely dangerous thing to do. It involves violence and abuse, and fundamentally effects the woman or child who is prostituted.

"With the hazards that prostitutes have to face daily, I would be extremely shocked if anybody who was involved for any period of time did not show signs of stress disorder."

Ms Bindel said the research was valuable because it demythologised prostitution, and revealed the idea of the "happy hooker" to be nonsense.